This invention relates to a variable capacity type hydraulic pump or motor and particularly to an axial piston system hydraulic motor provided with an axial cylinder for driving a vehicle.
In a vehicle which requires two travel speeds (e.g. for high and low speed travelling) such as a bulldozer or shoveldozer, a hydraulic circuit for travelling is accomplished by a series or parallel circuit, combined circuit or the like, or a hydraulic motor capable of varying the capacity by two steps is used as disclosed in the U.S. Pat. Nos. 4,037,521 and 4,048,903 or any means such as a hydraulic pump capable of varying the discharge by two steps are employed.
However, when the hydraulic circuit such as series circuit, parallel circuit or combined circuit is merely used, the circuit constitution becomes complicated and a lot of labor is needed for manufacture. Also, the two step change-over type hydraulic pump and hydraulic motor disclosed in the U.S. Pat. Nos. 4,037,521 and 4,048,903 specifications are large-sized and expensive by themselves and have disadvantages in that not only a special inconvenience is caused in mounting them on the bulldozer, shoveldozer or the like, but also the price of these vehicles is heightened.
Also, in addition, in order to hold these hydraulic pumps or motors at the variable position, since a variable operation mechanism must take on a piston thrust of the hydraulic pump or motor, the thrust of the variable operation mechanism has to be designed to be extremely large and thus the strength of parts constituting the variable operation mechanism becomes so large that this urges further this type of hydraulic pump or motor to be large-scaled and expensive.